The present invention relates to an optical disk drive and a super-resolution reproduction method for an optical disk.
Optical disk memories, which accomplish information reproduction alone or information recording and reproduction by irradiation of a light beam, have been put to practical use, as high-capacity, fast-access and portable storage media, in various files, such as audio data, image data and computer data. It is expected that development of those memories will continue. There may be several schemes available to increase the density of optical disks, such as shortening the wavelength of a gas laser for cutting a master, shortening the wavelength of a semiconductor laser as an operational light source, increasing the numerical aperture of an objective lens and making optical disks thinner. With regard to recordable optical disks, various other approaches are possible: mark length recording, and land/grove recording.
As a scheme having a great effect on improvement on the density of optical disks, a super-resolution reproduction technique which uses a medium film has been proposed. The super-resolution reproduction was originally proposed as a scheme specific to magneto-optical disks. In the super-resolution reproduction for magneto-optical disks, a magnetic film (super-resolution film) is provided on the reproduction-beam incident side with respect to the recording layer so as to cause exchange coupling or magnetostatic coupling between them. Then, a reproduction beam is irradiated to raise the temperature of the super-resolution film to change the exchange force or magnetostatic force, thereby forming an optical aperture or optical mask in the super-resolution film to realize super-resolution reproduction.
Later, for ROM disks in addition to MO disk, was reported an attempt to provide a super-resolution film whose light transmittance varies with the irradiation of a reproduction beam, on the reproduction-beam incident side with respect to the recording layer for the purpose of super-resolution reproduction. It has become obvious that super-resolution reproduction can be adapted to all optical disks like a magneto-optical disk, CD-ROM, CD-R, WORM and a phase change optical disk.
Implementation of super-resolution reproduction of optical disks requires that the transmittance of the super-resolution film should change by a significant amount with practical reproduction power, an optical aperture should be formed fast in as a short period as the pass time of the reproduction beam spot, and repeated reproduction should be accomplished stably. Prior arts cannot however meet all of those requirements.